1.0.0[][src]Trait std::default::Default

pub trait Default {
    fn default() -> Self;
}

A trait for giving a type a useful default value.

Sometimes, you want to fall back to some kind of default value, and don't particularly care what it is. This comes up often with structs that define a set of options:

struct SomeOptions {
    foo: i32,
    bar: f32,
}Run

How can we define some default values? You can use Default:

#[derive(Default)]
struct SomeOptions {
    foo: i32,
    bar: f32,
}

fn main() {
    let options: SomeOptions = Default::default();
}Run

Now, you get all of the default values. Rust implements Default for various primitives types.

If you want to override a particular option, but still retain the other defaults:

fn main() {
    let options = SomeOptions { foo: 42, ..Default::default() };
}Run

Derivable

This trait can be used with #[derive] if all of the type's fields implement Default. When derived, it will use the default value for each field's type.

How can I implement Default?

Provide an implementation for the default() method that returns the value of your type that should be the default:

enum Kind {
    A,
    B,
    C,
}

impl Default for Kind {
    fn default() -> Self { Kind::A }
}Run

Examples

#[derive(Default)]
struct SomeOptions {
    foo: i32,
    bar: f32,
}Run

Required Methods

Returns the "default value" for a type.

Default values are often some kind of initial value, identity value, or anything else that may make sense as a default.

Examples

Using built-in default values:

let i: i8 = Default::default();
let (x, y): (Option<String>, f64) = Default::default();
let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();Run

Making your own:

enum Kind {
    A,
    B,
    C,
}

impl Default for Kind {
    fn default() -> Self { Kind::A }
}Run

Implementors

impl Default for bool
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Returns the default value of false

impl Default for char
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Returns the default value of \x00

impl Default for f32
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Returns the default value of 0.0

impl Default for f64
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Returns the default value of 0.0

impl Default for i128
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Returns the default value of 0

impl Default for i16
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Returns the default value of 0

impl Default for i32
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Returns the default value of 0

impl Default for i64
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Returns the default value of 0

impl Default for i8
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Returns the default value of 0

impl Default for isize
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Returns the default value of 0

impl Default for u128
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Returns the default value of 0

impl Default for u16
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Returns the default value of 0

impl Default for u32
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Returns the default value of 0

impl Default for u64
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Returns the default value of 0

impl Default for u8
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Returns the default value of 0

impl Default for ()
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Returns the default value of ()

impl Default for usize
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Returns the default value of 0

impl Default for Global
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impl Default for Box<str>
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impl Default for Box<CStr>
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impl Default for Box<OsStr>
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impl Default for DefaultHasher
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Creates a new DefaultHasher using new. See its documentation for more.

impl Default for RandomState
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Constructs a new RandomState.

impl Default for CString
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Creates an empty CString.

impl Default for OsString
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Constructs an empty OsString.

impl Default for Error
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impl Default for SipHasher
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impl Default for PathBuf
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impl Default for String
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Creates an empty String.

impl Default for AtomicBool
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Creates an AtomicBool initialized to false.

impl Default for AtomicI16
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impl Default for AtomicI32
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impl Default for AtomicI64
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impl Default for AtomicI8
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impl Default for AtomicIsize
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impl Default for AtomicU16
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impl Default for AtomicU32
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impl Default for AtomicU64
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impl Default for AtomicU8
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impl Default for AtomicUsize
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impl Default for Condvar
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Creates a Condvar which is ready to be waited on and notified.

impl Default for Duration
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impl<'_> Default for &'_ str
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Creates an empty str

impl<'_> Default for &'_ mut str
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Creates an empty mutable str

impl<'_, T> Default for &'_ [T]
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Important traits for &'a [u8]

Creates an empty slice.

impl<'_, T> Default for &'_ mut [T]
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Important traits for &'a [u8]

Creates a mutable empty slice.

impl<'a> Default for &'a CStr
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impl<'a> Default for &'a OsStr
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Creates an empty OsStr.

impl<'a, B> Default for Cow<'a, B> where
    B: ToOwned + ?Sized,
    <B as ToOwned>::Owned: Default
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Creates an owned Cow<'a, B> with the default value for the contained owned value.

impl<A> Default for (A,) where
    A: Default
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impl<A, B> Default for (A, B) where
    A: Default,
    B: Default
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impl<A, B, C> Default for (A, B, C) where
    A: Default,
    B: Default,
    C: Default
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impl<A, B, C, D> Default for (A, B, C, D) where
    A: Default,
    B: Default,
    C: Default,
    D: Default
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impl<A, B, C, D, E> Default for (A, B, C, D, E) where
    A: Default,
    B: Default,
    C: Default,
    D: Default,
    E: Default
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impl<A, B, C, D, E, F> Default for (A, B, C, D, E, F) where
    A: Default,
    B: Default,
    C: Default,
    D: Default,
    E: Default,
    F: Default
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impl<A, B, C, D, E, F, G> Default for (A, B, C, D, E, F, G) where
    A: Default,
    B: Default,
    C: Default,
    D: Default,
    E: Default,
    F: Default,
    G: Default
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impl<A, B, C, D, E, F, G, H> Default for (A, B, C, D, E, F, G, H) where
    A: Default,
    B: Default,
    C: Default,
    D: Default,
    E: Default,
    F: Default,
    G: Default,
    H: Default
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impl<A, B, C, D, E, F, G, H, I> Default for (A, B, C, D, E, F, G, H, I) where
    A: Default,
    B: Default,
    C: Default,
    D: Default,
    E: Default,
    F: Default,
    G: Default,
    H: Default,
    I: Default
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impl<A, B, C, D, E, F, G, H, I, J> Default for (A, B, C, D, E, F, G, H, I, J) where
    A: Default,
    B: Default,
    C: Default,
    D: Default,
    E: Default,
    F: Default,
    G: Default,
    H: Default,
    I: Default,
    J: Default
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impl<A, B, C, D, E, F, G, H, I, J, K> Default for (A, B, C, D, E, F, G, H, I, J, K) where
    A: Default,
    B: Default,
    C: Default,
    D: Default,
    E: Default,
    F: Default,
    G: Default,
    H: Default,
    I: Default,
    J: Default,
    K: Default
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impl<A, B, C, D, E, F, G, H, I, J, K, L> Default for (A, B, C, D, E, F, G, H, I, J, K, L) where
    A: Default,
    B: Default,
    C: Default,
    D: Default,
    E: Default,
    F: Default,
    G: Default,
    H: Default,
    I: Default,
    J: Default,
    K: Default,
    L: Default
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impl<H> Default for BuildHasherDefault<H>
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impl<K, V> Default for BTreeMap<K, V> where
    K: Ord
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Creates an empty BTreeMap<K, V>.

impl<K, V, S> Default for HashMap<K, V, S> where
    K: Eq + Hash,
    S: BuildHasher + Default
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Creates an empty HashMap<K, V, S>, with the Default value for the hasher.

impl<T> Default for Option<T>
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Returns None.

impl<T> Default for [T; 0]
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impl<T> Default for [T; 1] where
    T: Default
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impl<T> Default for [T; 2] where
    T: Default
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impl<T> Default for [T; 3] where
    T: Default
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impl<T> Default for [T; 4] where
    T: Default
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impl<T> Default for [T; 5] where
    T: Default
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impl<T> Default for [T; 6] where
    T: Default
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impl<T> Default for [T; 7] where
    T: Default
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impl<T> Default for [T; 8] where
    T: Default
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impl<T> Default for [T; 9] where
    T: Default
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impl<T> Default for [T; 10] where
    T: Default
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impl<T> Default for [T; 11] where
    T: Default
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impl<T> Default for [T; 12] where
    T: Default
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impl<T> Default for [T; 13] where
    T: Default
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impl<T> Default for [T; 14] where
    T: Default
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impl<T> Default for [T; 15] where
    T: Default
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impl<T> Default for [T; 16] where
    T: Default
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impl<T> Default for [T; 17] where
    T: Default
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impl<T> Default for [T; 18] where
    T: Default
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impl<T> Default for [T; 19] where
    T: Default
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impl<T> Default for [T; 20] where
    T: Default
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impl<T> Default for [T; 21] where
    T: Default
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impl<T> Default for [T; 22] where
    T: Default
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impl<T> Default for [T; 23] where
    T: Default
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impl<T> Default for [T; 24] where
    T: Default
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impl<T> Default for [T; 25] where
    T: Default
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impl<T> Default for [T; 26] where
    T: Default
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impl<T> Default for [T; 27] where
    T: Default
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impl<T> Default for [T; 28] where
    T: Default
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impl<T> Default for [T; 29] where
    T: Default
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impl<T> Default for [T; 30] where
    T: Default
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impl<T> Default for [T; 31] where
    T: Default
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impl<T> Default for [T; 32] where
    T: Default
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impl<T> Default for Box<[T]>
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impl<T> Default for Box<T> where
    T: Default
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Important traits for Box<I>

Creates a Box<T>, with the Default value for T.

impl<T> Default for Cell<T> where
    T: Default
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Creates a Cell<T>, with the Default value for T.

impl<T> Default for RefCell<T> where
    T: Default
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Creates a RefCell<T>, with the Default value for T.

impl<T> Default for UnsafeCell<T> where
    T: Default
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Creates an UnsafeCell, with the Default value for T.

impl<T> Default for Reverse<T> where
    T: Default
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impl<T> Default for BinaryHeap<T> where
    T: Ord
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Creates an empty BinaryHeap<T>.

impl<T> Default for BTreeSet<T> where
    T: Ord
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Makes an empty BTreeSet<T> with a reasonable choice of B.

impl<T> Default for LinkedList<T>
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Creates an empty LinkedList<T>.

impl<T> Default for VecDeque<T>
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Creates an empty VecDeque<T>.

impl<T> Default for Empty<T>
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impl<T> Default for PhantomData<T> where
    T: ?Sized
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impl<T> Default for ManuallyDrop<T> where
    T: Default + ?Sized
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impl<T> Default for Wrapping<T> where
    T: Default
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impl<T> Default for Rc<T> where
    T: Default
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Creates a new Rc<T>, with the Default value for T.

Examples

use std::rc::Rc;

let x: Rc<i32> = Default::default();
assert_eq!(*x, 0);Run

impl<T> Default for std::rc::Weak<T>
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Constructs a new Weak<T>, allocating memory for T without initializing it. Calling [upgrade][Weak::upgrade] on the return value always gives None.

Examples

use std::rc::Weak;

let empty: Weak<i64> = Default::default();
assert!(empty.upgrade().is_none());Run

impl<T> Default for AtomicPtr<T>
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Creates a null AtomicPtr<T>.

impl<T> Default for Arc<T> where
    T: Default
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Creates a new Arc<T>, with the Default value for T.

Examples

use std::sync::Arc;

let x: Arc<i32> = Default::default();
assert_eq!(*x, 0);Run

impl<T> Default for std::sync::Weak<T>
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Constructs a new Weak<T>, without allocating memory. Calling [upgrade][Weak::upgrade] on the return value always gives None.

Examples

use std::sync::Weak;

let empty: Weak<i64> = Default::default();
assert!(empty.upgrade().is_none());Run

impl<T> Default for Vec<T>
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Important traits for Vec<u8>

Creates an empty Vec<T>.

impl<T, S> Default for HashSet<T, S> where
    T: Eq + Hash,
    S: BuildHasher + Default
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Creates an empty HashSet<T, S> with the Default value for the hasher.

impl<T: Default> Default for RwLock<T>
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Creates a new RwLock<T>, with the Default value for T.

impl<T: ?Sized + Default> Default for Mutex<T>
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Creates a Mutex<T>, with the Default value for T.